Ferroelectric optical crystals such as a LiNbO3 (LN) crystal, a KTiOPO4 (KTP) crystal, a LiB3O5 (LBO) crystal, and a β-BaB2O4 (BBO) crystal are mainstreams of a material used in an optical device where a secondary nonlinear optical phenomenon is employed. The optical device using these crystals has been developed in wide application fields based on wavelength conversion. For example, the optical device is used in a laser processing field, an optical communication field, and a measuring field.
In the laser processing field, the optical device is shortened in wavelength using a second harmonic generation (SHG) of an optical fiber laser. Since the diameter of a beam spot of the second harmonic becomes smaller than that of a fundamental waveform, the optical device is used in a fine laser processing. In the optical communication field, the optical device is used to integrally convert the wavelengths of a C-band WDM signal to an L-band WDM signal in order to effectively utilize a wavelength resource of the WDM signal. In the measuring field, the optical device is used as a light source which generates a terahertz light beam to observe intermolecular vibrations caused by hydrogen bonding.
Recently, compound semiconductor crystals such as GaAs, CaP, GaN, CdTe, ZnSe, and ZnO are also used as a material of the optical device which uses the secondary nonlinear optical phenomenon. These materials have a large secondary nonlinear optical constant and as well draw attention as a material of a secondary-order nonlinear device as there is developing a technology of manufacturing a periodically-poled structure which is essential in a secondary nonlinear optical device.
A scheme of the wavelength conversion may be classified into two fields of a quasi-phase matching (QPM) on the basis of the periodically poling and an angle phase matching. The quasi-phase matching in these fields can generate various phase matching wavelengths by designing an appropriate poling pitch. The wavelength conversion can be made over the entire transparent region of the material. In addition, since the quasi-phase matching has no work-off angle caused by the angle phase matching, a beam quality is excellent, and an interaction length can be made long. Therefore, the quasi-phase matching is a method which is suitable for a high efficiency and for suppressing a coupling loss, and is effective in processing and measuring.